The present invention relates to a device and to a method for adjusting the buoyancy of risers used for connecting a subsea wellhead to a floating drilling support.
The risers used in the profession consist of tubular elements whose length ranges between 15 and 25 m (50 and 80 feet), linked together by connectors. The weight of these risers can be very great, which requires high-capacity suspension means at the surface. Furthermore, the stresses resulting from external loads on such a heavy element are high. It is therefore essential to decrease the apparent weight of these risers with lightening means. Known devices consist of buoy-type elements made from a light material and withstanding the hydrostatic pressure, sealed bottles filled with gas or buoyancy boxes comprising devices for filling them with air according to a predetermined surface adjustment.
The aforementioned device can be illustrated by document FR-2,314,347, which describes annular boxes concentric to a riser element comprising lower openings for water inflow and a device provided with a float for adjusting the water level, therefore the buoyancy level, in said box. This device does not allow buoyancy adjustment when the riser elements are assembled in the water depth.
Water depths can now reach 3000 m, which requires optimized remote-controlled buoyancy means.
The present invention thus relates to a riser for drilling with subsea wellhead comprising tubular elements linked together by connecting devices, the elements comprising a buoyancy device consisting of a box in which a determined volume of gas can be pumped so as to modify the apparent weight of said element in the water. At least one tubular element comprises means for measuring the differential pressure between the inside and the outside of said box, means for filling the box with gas, means for discharging the gas from the box, means for controlling the filling means and the discharge means considering said differential pressure measurement.
The tubular element of the riser can comprise means for receiving orders to fill or to empty said box.
The element can also comprise means for transmitting the differential pressure measurement to the surface.
The gas supply means can consist of pipes parallel to said tubular elements and linked together by said connecting devices.
The energy required for control can be provided by a hydraulic line similar to the gas supply line.
In the riser, at least one electric conductor can connect the lower and upper connector of a tubular element, and the connectors can link together the conductors of the various tubular elements.
The invention further relates to a method for adjusting the buoyancy of a riser for drilling with subsea wellhead comprising tubular elements linked together by connecting devices, said elements comprising each a floating device consisting of a box in which a determined volume of gas can be pumped so as to modify the apparent weight of said element in the water.
According to the method, the buoyancy of said element is calculated by measuring the differential pressure between the inside and the outside of the box, means for filling said box with gas or for emptying it from gas are controlled according to the desired buoyancy.
Control orders can be sent from the surface to at least one tubular element comprising means for receiving said orders in connection with the means for controlling the gas filling or discharge means.